Low friction cobalt based coatings for titanium alloys

ABSTRACT

A cobalt based coating is provided which is functional to reduce the coefficient of friction between two operatively, frictionally engaging titanium alloy surfaces.

This Application is a divisional application of application Ser. No.08/504,638, filed Jul. 20, 1995, now U.S. Pat. No. 5,601,933, which is acontinuation-in-part of application Ser. No. 08/214,263 filed Mar. 17,1994, now abandoned.

FIELD OF THE INVENTION

The present invention relates to protective coatings adapted fortitanium based alloy parts which come into frictional engagement withone another resulting in metal-to-metal wear. More specifically, thecoatings find particular application in the protection from adhesive andfretting wear and galling and seizure of gas turbine and jet engineparts or the like made from titanium and titanium alloys.

BACKGROUND OF THE INVENTION

In certain applications, titanium based alloys are utilized underconditions wherein there is frictional metal-to-metal contact betweentwo titanium alloy surfaces. An exemplary use would be that between thefrictional parts of the roots of the compressor blades against the discsof a jet engine. The blades are secured to their respective discs bymeans of a dovetail arrangement in which the dovetail base of a bladefits into a groove formed on the periphery of a rotating disc. Duringoperation of the jet engine, centrifugal forces and vibration cause theblade to move in the groove. The fit of the dovetail base portion of theblade in the disc groove is designed to allow for this movement.

Local aerodynamic effects, resulting from the vane or strut positionsrelative to the rotating components in the compressor, will produce highfrequency, low amplitude vibrations. The combination of high contactstress due to the centrifugal forces and high frequency vibration causesextensive surface damage to unprotected surfaces. Stated otherwise,maintenance of direct metal to metal contact between two titanium alloysurfaces leads to a high coefficient of friction with serious damage toboth surfaces due to metal-to-metal wear of the metal alloy.

Generally, it is the fan and the front stage compressor blades operatingunder relatively low temperatures and which are manufactured fromlight-weight titanium based alloys which are prone to metal-to-metalwear, whereas the rear stage compressor components operating underrelatively higher temperatures are formed from materials such as steelor nickel based alloys which are less prone to metal-to-metal wear.

The current commercial practice involves protecting the titanium alloycomponents of gas turbine and jet engines by the application of athermal spray coating, preferably in conjunction with a solid filmlubricant, to the dovetail base of the blade. The protective coating iscomposed of a soft copper-nickel alloy or a copper-nickel-indium alloy,which is applied by plasma spraying. Subsequently, the coating is dryfilm lubricated using molybdenum disulfide in an organic resin binder.This solid film lubricant initially provides a low coefficient offriction in an attempt to prevent galling, and to delay the onset ofadhesive wear originating from metal-to-metal contact. This is describedin ASM Handbook, Volume 18, October 1992, pp. 588-592 by Schell, J. D.et al.

Once this solid lubricant has been eroded, the CuNiIn coating on thedovetail base of the blade rubs against the titanium alloy disc groovesurfaces damaging the disc by removal of the metal from the disc and bycreating pits in the disc. The disc damage may lead to seizure of theblades within the disc grooves or, in extreme cases, to prematurefatigue failure of the disc.

Thus, these prior art coatings have not proved fully successful in theprevention of the metal-to-metal wear of surfaces of titanium alloys infrictional contact with one another. Disadvantageously, too, suchcopper-nickel-indium and copper-nickel alloy systems display a goodoxidation resistance up to about 315° C. Usually, jet engines and thelike are subjected to a broad temperature gradient in the fan/compressorwhich ranges from sub-zero (-60° C.) up to approximately 600° C. at therear compressor stage, substantially above the effective 315° C. uppertemperature limit of the copper-nickel and copper-nickel-indium alloys.

It is well known in the art to utilize cobalt based alloys for overlaycoatings on substrates to reduce oxidation and corrosion at elevatedtemperatures. In the U.S. Pat. No. 4,034,142 issued Jul. 5, 1977 to R.J. Hecht, there are disclosed overlay coatings for use exclusively inhigh temperature applications for the protection of substrates againstoxidation and hot corrosion. The coatings contain aluminum, chromium,yttrium and silicon and a metal chosen from the group consisting ofnickel, cobalt and iron or mixtures thereof. The coatings areparticularly suited for the protection of nickel and cobalt superalloysat elevated temperatures, i.e. of the order of 1000° C., by theformation of a stable oxide surface layer of alumina on the coatingswhich acts as a diffusion barrier to minimize further reactions.

A. R. Nicoll, in U.S. Pat. No. 4,503,122 issued Mar. 5, 1985, provides ahigh temperature protection layer for temperatures above 600° C.,usually substantially above 900° C., for high temperature gas turbineparts manufactured from an austenitic material such as a nickelsuperalloy. The layer is composed of a base of chromium, aluminum andcobalt with silicon and yttrium. Again, the protective nature of thecoatings relies on the formation of a continuous cover of an aluminaskin resistant to high temperature corrosion at above 900° C.

W. J. Brindley et al, U.S. Pat. No. 5,116,690 issued May 26, 1992discloses overlay coatings of MCrAlX in which M may be nickel, cobalt oriron and X may be yttrium, Yb, Zr, or Hf on Ti₃ Al+Nb titanium alloys inan oxidizing environment.

The overlay coatings disclosed in the Hecht, Nicoll and Brindley et alpatents are used at temperatures in excess of 900° C., well above thetemperatures at which titanium alloys are used. The overlay coatingsprotect the airfoil of a blade against destructive influence of hot gasatmospheres. No protection for wear due to rubbing of an overlay coatingagainst another solid surface is contemplated.

U.S. Pat. No. 4,789,441 issued Dec. 6, 1988 to J. Foster et al and U.S.Pat. No. 4,810,334 issued Mar. 7, 1989 to F. J. Honey et al discloseprotection layers on turbine blades comprised of particles of chromium,aluminum, yttrium and silicon in a matrix of cobalt applied by compositeelectrolytic deposition. The protective layers and anchoring coats of alarger particle size are applied by electrolytic deposition and thenspray coated with a thermal barrier of a refractory material by plasmadeposition. These patents thus relate to the use of a cobalt alloy toanchor a ceramic coating to a substrate.

Privett III, et al. in U.S. Pat. No. 5,292,596 issued Mar. 8, 1994provides a method for protecting a force-transmitting or force-receivingsurface of titanium from fretting fatigue. The composition of thecoating used is essentially, by weight, 30 to 70% cobalt, about 25 to55% nickel and about 5 to about 25% iron. The essential feature of thepatent disclosure is the presence of iron which, when oxidized tohematite at elevated temperatures, in the range of 480 to 650° C.provides the improved anti-fretting properties.

Luthra et al. in U.S. Pat. No. 5,077,140 issued Dec. 31, 1991 disclose amethod for protecting substrates from oxidation at temperatures of up toabout 900° C. The coatings consist of a continuous coating of ductileMCrAl or ductile MCr alloys where M is at least one metal selected fromthe group consisting of iron, nickel and cobalt.

Luthra et al.'s coatings protect titanium substrates against oxidationand not against metal-to-metal wear. According to Luthra et al., thecoatings are useful for temperatures above 600° C. when the titaniumalloys have a high affinity for oxygen. Luthra et al. coatings do notprovide metal-to-metal wear protection and, furthermore, they areintended only for elevated temperatures in a restricted range of600-900° C.

Cobalt alloy coatings useful as wear-resistant materials are exemplifiedin such products as "Stellite"* and "Tribaloy"*. The Stellite typecoatings are typically composed of a chromium, tungsten strengthenedcobalt matrix containing a high percentage of very hard carbides,predominantly chromium carbides. The Tribaloy type coatings are eithercobalt or nickel based with molybdenum, silicon and chromium as themajor alloying elements. The Tribaloy compositions are so balanced thatthe bulk of the structure is in hard, brittle, laves phases having aRockwell Hardness (HRC) in the 50 to 60 range. Both Stellite andTribaloy alloys are so hard as to prove unmachinable, and it is thishardness which is responsible for the wear resistant properties ofcoated articles. Unfortunately, should a softer part, such as one formedfrom titanium based alloy, be rubbed against by the coating-hardenedsurface, the latter will cause the softer part to wear excessively.

Cobalt based superalloys have been developed for their high temperaturemechanical properties and oxidation resistance. They are typically usedas wrought or cast solid parts, not as a coating. Although such alloyswill not damage other superalloy surfaces when in contact with them,they cannot be used with titanium alloys without deleterious effectsthereto.

*Trademarks

SUMMARY OF THE INVENTION

In accordance with the present invention, there are provided protectivecoatings particularly useful to reduce the coefficient of friction andto enhance wear-resistance of titanium based alloys.

In its simplest aspect, the protective coating comprises a film ofsubstantially pure cobalt which may be applied such as by thermalspraying onto one of the friction engaging titanium alloy surfaces or byinterposing a thin film of cobalt material between a pair of frictionengaging titanium alloy surfaces whereby the protective cobalt materialis transferred onto one or both friction engaging surfaces by rubbingcontact.

In a second broad aspect, the protective coating additionally comprisescobalt together with one or more of chromium, aluminum, silicon,yttrium, hexagonal boron nitride or selective mixtures thereof.

More particularly, the present invention comprises a mechanicalcomponent of titanium or titanium alloy having a contacting surface foruse in an apparatus having an opposed, contacting surface of titanium ortitanium alloy, which in operation subjects the surface of saidmechanical component to fretting adhesive wear, i.e. metal-to-metalwear, comprising the said surface of said mechanical component having anadherent, protective, lubricous coating comprising cobalt or cobalt andat least one alloying element selected from the group consisting ofchromium, aluminum, silicon and yttrium in an amount effective forcontact transfer of a portion of the adherent, lubricous, protectivecoating from the surface of the mechanical component to the opposed,contacting surface of the apparatus whereby the surfaces are protectedfrom fretting wear. Typically, the mechanical component is a jet enginecomponent of titanium alloy, which in operation has a surface that issubjected to metal-to-metal wear in a temperature range of -60° C. to600° C., said surface having a said protective, adherent, lubricouscoating formed thereon such as by thermal spraying. The protectiveadherent, lubricous coating may comprise one or more of cobalt, cobaltand about 0.01 to about 35% by weight chromium, cobalt and about 0.01 toabout 20% by weight aluminum, cobalt and about 0.01 to about 7% byweight silicon, cobalt and about 0.01 to about 2% by weight yttrium, andadditionally hexagonal boron nitride in the range of about 0.01 to about30% by volume.

A preferred cobalt alloy composition range comprises, by weight, 17-35%chromium, 3-12.5% aluminum, 0.5-7% silicon, 0.3-2% yttrium, the balancecobalt. Another preferred cobalt alloy composition comprises, by weight,29.2% chromium, 6.1% aluminum, 2% silicon, 0.35% yttrium, the balancecobalt.

A further preferred cobalt alloy composition comprises, by weight, 6-12%aluminum and the balance cobalt. Another preferred cobalt alloycomposition comprises, by weight, about 9% aluminum and the balancecobalt.

Another preferred aspect of the invention contemplates the addition ofhexagonal boron nitride to each specific coating formulation. It ispostulated that there is uniform distribution of the boron nitridethroughout the coating which results in improved lubrication.

Advantageously, the coatings disclosed herein prevent destructivefrictional contact between the surfaces of titanium alloy components. Itis to be emphasized that the coatings are specific to titanium alloys.Furthermore, because of the low coefficient of friction in comparison tothat of the uncoated titanium alloys, minimal disturbance of theuncoated titanium alloy surfaces occurs. Thus, the resultant diminutionin scratching and pitting of the titanium alloy surface normally due tometal-to-metal wear results in reduced risk of fatigue cracking.Additionally, the inclusion of the alloying elements in certain coatingsprovides high temperature capability. A marked advantage of the coatingsdisclosed herein lies in the fact that their protective properties areinherent at ambient temperatures, unlike the prior art overlay coatingswhich require the application of heat before becoming functional.

It is to be noted also that the coatings exhibit predictable and stablelong term wear and frictional behaviour. Also, the coatings are longerlasting than those of the prior art and require less maintenance.Somewhat surprisingly, it has been observed that the coefficient offriction appears to decrease with time as opposed to the prior artcoatings, which have an increasing coefficient of friction with time.

In summary, therefore, the invention provides a novel coated mechanicalcomponent formed of titanium alloy substrate coated with a cobalt orcobalt alloy based coating which is capable of frictionally engaging asecond component which is also formed of titanium based alloy fortransfer of the cobalt or cobalt alloy to untreated surfaces of themechanical components by rubbing contact between the merchanicalcomponents whereby the coefficient of friction therebetween is markedlyreduced, with concomitant damage to both titanium alloy components beingsubstantially eliminated.

As will be evident to one skilled in the art, the present discoveryprovides a method of reducing damage to the surfaces of two frictionallyengaging alloy surfaces, wherein the alloy is specifically titaniumbased.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating the applied coating asutilized in the context of a jet engine.

FIG. 2 is a plot showing the decrease of the coefficient of frictionversus time for two coating formulations in comparison to that of aprior art coating.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Having reference to the accompanying drawings wherein the application ofthe invention specifically to jet engines is demonstrated, there isshown in FIG. 1 a single compressor blade 10. The compressor disc 12 hasa groove 14 formed on the periphery thereof adapted to receive the rootor base 16 of blade 10 in a dovetail engagement. The blade root 16 orthe inner shoulders of the disc groove 14 are thermally sprayed withcoating 20, as illustrated. Whilst the invention is in part describedwith respect to a jet engine, it is to be understood that the inventionis not to be limited to this application, but can be applied whenevertwo titanium or titanium alloy components come into frictionalengagement with one another. Other techniques such as physical vapourdeposition, chemical vapour deposition and the like can be used todeposit the coating.

The experimental data generated and given herebelow were obtained byapplying the coating using standard high velocity oxy-fuel sprayingequipment. In the spraying process a powder of the alloy to be appliedis introduced into a hot supersonic plume formed by burning a combustiongas, for example, propylene, mixed with oxygen. The powder particles areheated and accelerated in the plume. A portion of the heated particlesadheres to the titanium alloy to form the continuous protective coating20. The coating thickness preferably is approximately 127 micrometersbut is adjustable by controlling spray parameters, as is well known inthe art.

EXAMPLE 1

Cobalt powder having a particle size less than 45 micrometers andcomprising essentially cobalt with 0.69% oxygen, was sprayed onto aTi-6Al-4V sample. The titanium alloy sample had been previouslydegreased and grit blasted using 40 grit (450 micrometers) alumina at apressure of 550 kPa using a suction type grit blaster.

The spraying was effected using an air cooled high velocity oxy-fuel(HVOF) unit. The spray conditions were as follows:

    ______________________________________    Oxygen flow   20,400             litres/hour    Propylene flow                  4,200              litres/hour    Spray distance                  20.3               cm    Powder flow rate                  16                 g/min    ______________________________________

The coating was sprayed to a 127 um thickness which is typical of theactual application. The coatings had the following mechanicalproperties: the diamond pyramid hardness load 300 g (DPH₃₀₀) was 228 andthe ultimate tensile strength (U.T.S.) was 63.4 MPa. The coating oxygencontent was 2%.

The testing of the coating was carried out at room temperature in awear-friction rig simulating the conditions encountered in thefrictional contact between a dovetail blade attachment in a jet enginedisc. The coating was rubbed against a Ti-6Al-4V block with thefollowing results:

                  TABLE 1    ______________________________________    Initial Coefficient of Friction                         0.46    Final Coefficient of Friction                         0.30    Stability of Coefficient of Friction                         slightly                         variable    Ti-6AL-4V block material pick-up                         18 um    Deepest Ti-6A1-4V block pit                          0 um    Coating Wear         89 um    ______________________________________

EXAMPLE 2

Cobalt alloy powder having a particle size less than 45 micrometers wasused. The powder had the composition given below:

    ______________________________________           Element                 Weight %    ______________________________________           Cr    28.9           Al    5.8           Si    1.7           Y     0.35           Co    Balance    ______________________________________

The titanium alloy substrate, its preparation, spray equipment and sprayparameters were similar to those given in Example 1 to yield a coatingof 127 um thickness. The coating had the following mechanicalproperties: the DPH₃₀₀ was 650 and the U.T.S. was 54.1 MPa. The testresults are set out herebelow:

                  TABLE 2    ______________________________________    Initial Coefficient of Friction                         0.59    Final Coefficient of Friction                         0.36    Stability of Coefficient of Friction                         slightly                         variable    Ti-6A1-4V block material pickup                         12 um    Deepest Ti-6A1-4V block pit                          0 um    Coating wear         58 um    ______________________________________

EXAMPLE 3

A mechanical mixture of the CoCrAlSiY alloy powder having thecomposition described in Example 2 had 30 weight % hexagonal boronnitride (BN) of approximately 100 micrometer average particle size addedthereto. The titanium alloy preparation, spray equipment and sprayparameters were similar to those given in Example 1 to yield a coatingof 127 um thickness.

The coating was found to have a DPH₃₀₀ of 330 and a U.T.S.of 39 MPa. Thetest results are given below:

                  TABLE 3    ______________________________________    Initial Coefficient of Friction                         0.49    Final Coefficient of Friction                         0.36    Stability of Coefficient of Friction                         stable    Ti-6A1-4V block material pick-up                         12 um    Ti-6A1-4V block deepest pit                          0 um    Coating wear         79 um    ______________________________________

X-ray analysis indicated the presence of hexagonal BN in the coating.Metallographic examination showed a uniform distribution of BNthroughout the thickness of the coating. Chemical analysis indicatedabout 17 volume % of hexagonal BN in the coating.

It was determined that the addition of BN softened the coating comparedto the coating without BN (Example 2). BN also caused the initialcoefficient of friction to be lowered and in general the coefficient offriction was more stable during rubbing.

EXAMPLE 4

A CuNiIn coating having a comparable chemical composition to thatcommonly used in the prior art was applied to the same titanium alloyand subjected to identical tests as those conducted on the coatingsdescribed above.

The chemical composition of the CuNiIn coating was approximately: 58%Cu, 37% Ni, 5% In. The data for the CoCrAlSiY-BN coating are the same asfor Example 3.

                  TABLE 4    ______________________________________                 CoCrAlSiY--BN                           CuNiIn Coating                 Coating   As sprayed    ______________________________________    Initial Coefficient of                   0.49        0.3    Friction    Final Coefficient of                   0.36        0.58    Friction    Stability of Coefficient of                   stable      unstable,    Friction                   increases                               with time    Ti-6AL-4V block wear                   12 um (pick-up)                               58 um (wear)    or pick-up    Deepest Ti-6A1-4V                    0 um       127 um    block pit    Coating wear   79 um        86 um    Coating deepest pit                   99 um       107 um    ______________________________________

The superior performance of the CoCrAlSiY-BN coating was demonstratedby:

(i) the observation that the CoCrAlSiY-BN coating material transferredfrom the coated substrate to the titanium alloy block provided wearprotection thereto. The CuNiIn coated item damaged the titanium alloyblock extensively by removing a deep layer and pitting of the blockmaterial.

(ii) a lower and stable coefficient of friction, as shown in FIG. 2.

(iii) very reproducible performances in the wear and friction rig asdemonstrated with the CoCrAlSiY-BN coatings sprayed with differentbatches of CoCrAlSiY and BN powders.

(iv) the coating wear depth and the maximum pitting depth were 92% and93% of the CuNiIn coating, compared to 60% and 67% respectively of thedepth of CoCrAlSiY without BN addition relative to CuNiIn.

EXAMPLE 5

Comparative tests were conducted on the test coatings shown in Table 5below according to the method described in Example 1 above attemperatures of 24° C. and 454° C. Test results are shown in Tables 5and 6.

                  TABLE 5    ______________________________________                  Ti/6Al/4V Block                              Shoe Coating              Coefficient                        Wear(-), or                                  Deepest    Deepest    Test      of Friction                        Pickup(+) Pit   Wear Pit    Coating   Initial                     Final  μm   μm μm                                               μm    ______________________________________    Sliding Wear Test at 454° C.    CoCrAlYSi/BN              0.59   0.42   +10     0     -8   15    CoCrAlYSi/BN              0.50   0.35   +8      33    -81  145    NiCrMoNbFe/BN              0.49   0.48   -140    200   -30  50    NiCr/BN   0.53   0.61   -33     66    -10  23    TiAl/BN   0.51   0.61   -195    310   -246 361    Sliding Wear Test at 24° C.    Co/CoO    0.46   0.30   +18     0     -89  114    CoCrAlYSi 0.59   0.36   +12     0     -58  72    CoCrAlYSi/BN              0.49   0.36   +12     0     -79  99    CuNiIn    0.30   0.58   -58     127   -86  107    COCrAlNiY 0.56   0.48   0       10    -68  122    CoCrNiWSiC*(1)              0.47   0.67   -56     91    -91  157    CoCrMoSi(2)              0.77   0.55   -218    315   -56  86    CoAl      0.40   0.27   +5      2     -63  81    CoCr      0.62   0.37   +13     0     -68  89    CoO       0.64   0.66   -86     114   -152 218    ______________________________________     *Test stopped prematurely due to seizure of rubbing surfaces     (1)STELLITE ™ Composition     (2)TRIBALOY ™ Composition

                                      TABLE 6    __________________________________________________________________________    Test    Composition, wt %   U.T.S.                                    Microhardness    Coating Coating Alloy     BN                                MPa DPH.sub.300    __________________________________________________________________________    Sliding Wear Test at 454° C.    CoCrAlYSi/BN            63.2 Co, 28.9 Cr, 5.8 Al, 0.4 Y, 1.7 Si                              15                                na  na    CoCrAlYSi/BN            63.2 Co, 28.9 Cr, 5.8 Al, 0.4 Y, 1.7 Si                              30                                38-41                                    335    NiCrMoNbFe/BN            63.7 Ni, 21.6 Cr, 9 Mo, 3.7 Nb, 2 Fe                              15                                na  na    NiCr/BN 80 Ni, 20 Cr      9 na  na    TiAl/BN 100 TiAl          20                                na  na    Sliding Wear Test at 24° C.    Co/CoO  98 Co, 2 O.sub.2  0 63.4                                    228    CoCrAlYSi            63.2 Co, 28.9 Cr, 5.8 Al, 0.4 Y 1.7 Si                              0 54.1                                    650    CoCrAlYSi/BN            63.2 Co, 28.9 Cr, 5.8 Al, 0.4 Y, 1.7 Si                              30                                39.0                                    330    CuNiIn  58 Cu, 37 Ni, 5 In                              0 na  na    COCrAlNiY            58.7 Co, 23.9 Cr, 7.1 Al, 9.8 Ni, 0.5 Y                              0 61.6                                    na    CoCrNiWSiC*            52.9 Co, 27 Cr, 11 Ni, 8 W, 0.6 Si, 0.5 C                              0 58.5                                    na    CoCrMoSi            50.6 Co, 17.5 Cr, 28.5 Mo, 3.4 Si                              0 51.6                                    na    CoAl    91 Co, 9 Al       0 61.5                                    na    CoCr    80 Co, 20 Cr      0 63.4                                    na    CoO     100 CoO           0 53.9                                    na    __________________________________________________________________________

The 24° C. Sliding Wear Test data obtained by rubbing a shoe ofTi-6Al-4V having protective coatings of a thickness of about 127micrometers thereon against an uncoated block of Ti-6Al-4V show clearlythat cobalt metal and cobalt alloys of the invention containing Cr, Al,Si and Y all function similarly in that coating material was transferredto the uncoated Ti-6Al-4V block and pitting damage to the uncoated blockwas virtually eliminated under the test conditions.

Protective coatings comprised of CoCrAlYSi/BN produced 10 micron and 8micron transfers (pick-up) of the coating to the uncoated surface andyielded significant coefficient of friction reductions of 0.017 to0.015, i.e. from 0.59 to 0.42 and from 0.50 to 0.35, at 454° C.

The three high temperature tests on non-cobalt alloys (nickel andtitanium alloys) containing BN had essentially unchanged or increasedcoefficients of friction, and up to 195 micron losses of titanium alloysurface materials with up to 310 micron deep pits in the uncoatedsurfaces were observed.

Protective coatings comprised of Co/Co0 (2% by weight oxygen largelyresulting from oxidation during the spray coating process), CoCrAlYSi,CoCrAlYSi/BN, CoAl and CoCr produced 5 micron to 18 micron transfers ofthe coatings to the uncoated surfaces and yielded low final coefficientsof friction in the range of 0.27 to 0.37, at 24° C.

The addition of Ni, W, Si, C, or Mo to the cobalt-chrome alloy baseclearly had a deleterious, and in one case, disastrous effect on thebehaviour of the coating. The effect of nickel itself as a substitutefor Si was definitely negative (see the results for the CoCrAlNiYalloy). The negative results on TRIBALOY™ (CoCrMoSi) and STELLITE™(CoCrNiWSiC) coatings are significant, particularly the severe wear of218 micron loss of uncoated surface and 315 micron deep pitting of theuncoated surface by the TRIBALOY coating. These tests confirm theimportance of selecting the right alloying elements to add to the Cobase alloy.

EXAMPLE 6

Cobalt alloy powder as described in Example 2 was mixed with a graphitepowder having a particle size in the -74 um+44 um range. The two powderswere mixed in a 1:1 weight ratio before spraying.

The titanium alloy substrate, its preparation, spray equipment and sprayparameters were similar to those given in Example 1 to yield a coatingof 127 um thickness. The coating had the following mechanicalproperties: the DPH₃₀₀ was 290 and the U.T.S. was 31.6 MPa.

The test results are shown in Table 7 below.

                  TABLE 7    ______________________________________    Initial Coefficient of Friction                             0.56    Final Coefficient of Friction                             0.73    Stability of Coefficient of Friction                             very                             unstable    Ti-6A1-4V block material wear                              27.9 um    Deepest Ti-6A1-4V Block Pit                              48.3 um    Coating Wear             239 um    ______________________________________

The presence of graphite in the coating was confirmed by X-Raydiffraction. A chemical analysis indicated approximately 13 vol % ofgraphite in the structure. The coefficient of friction was very unstableand increased during the test. No coating pick-up was observed and blockmaterial wear and pitting were significant.

EXAMPLE 7

Co-9 Al alloy powder was gas atomized and screened to a size range of-44 um. The titanium alloy substrate, its preparation, spray equipmentand spray parameters were similar to those given in Example 1 to yield acoating of 127 um thickness. The coating had U.T.S. of 67.0 MPa. Thetest results are set out herebelow:

                  TABLE 8    ______________________________________    Initial Coefficient of Friction                         0.37    Final Coefficient of Friction                         0.26    Stability of Coefficient of Friction                         Very stable    Ti-6A1-4V Block Material Pick-up                          9 um    Deepest Ti-6A1-4V Block Pit                          0 um    Coating Wear         51 um    ______________________________________

FIG. 2 illustrates the low and stable coefficient of friction obtainedfrom the CoAl coating compared to the CuNiIn coating. The coefficient offriction significantly descreased during the test from 0.37 to 0.26.Block material pick-up and no pitting were observed.

The present invention provides a number of important advantages. Aprotective coating for reducing the coefficient of friction and forenhancing wear resistance of titanium based alloys, said protectivecoating comprised of cobalt or cobalt based alloys containing at leastone of chromium, aluminum, silicon, yttrium or hexagonal boron nitride,can be applied to either one of a pair of opposed, friction engagingsurfaces by rubbing contact of a film of the cobalt or cobalt basedalloy interposed between the opposed surfaces. Preferably, one of a pairof opposed surfaces in rubbing engagement is adherently coated with aprotective film such as by thermal spraying and the coating filminterposed between the opposed surfaces is transferred onto the opposedunprotected surface by rubbing contact. Alternatively, a thin film ofthe protective cobalt based material can be placed between the opposed,friction engaging surfaces for transfer of a coating onto both of thesaid opposed surfaces by rubbing contact.

It will be understood that changes and modifications may be made in theembodiments of the invention without departing from the scope andpurview of the appended claims.

We claim:
 1. A method for protecting titanium and titanium alloycomponents of an apparatus having a pair of opposed, contacting surfaceswhich, in operation, are subjected to metal-to-metal wearcomprising:applying to one of said pair of opposed, contacting surfacesa lubricous, protective film consisting essentially of cobalt and atleast one alloying element selected from the group consisting, byweight, of about 17 to about 35% chromium, about 3 to about 12.5%aluminum, about 0.5 to about 5% silicon and about 0.3 to about 2%yttrium for contact transfer of a portion of the lubricous, protectivecoating from the coated surface to the opposed, contacting surfacewhereby the surfaces are protected from metal-to-metal wear.
 2. A methodfor protecting titanium and titanium alloy components of an apparatushaving a pair of opposed, contacting surfaces which in operation aresubjected to metal-to-metal wear comprising:thermal spraying onto one ofsaid pair of opposed, contacting surfaces an adherent lubricous,protective coating consisting essentially of cobalt and at least onealloying element selected from the group consist by weight about 17 toabout 35% chromium, about 3to about 12.5% aluminum, about 0.5 to about5% silicon and about 0.3 to about 2% yttrium for contact transfer of aportion of the lubricous, protective coating from the coated surface tothe opposed, uncoated, contacting surface whereby the surfaces areprotected from metal-to-metal wear.
 3. A method as claimed in claim 2 inwhich the adherent, lubricous, protective coating consists essentiallyof cobalt and about 27 to about 32% by weight chromium.
 4. A method asclaimed in claim 3 in which the adherent, lubricous, protective coatingincludes about 4 to about 8% by weight aluminum.
 5. A method as claimedin claim 4 in which the lubricous, protective coating includes about 1.0to about 3% by weight silicon.
 6. A method as claimed in claim 5 inwhich adherent, lubricous, protective coating includes about 0.35 toabout 1.0% by weight yttrium.
 7. A method as claimed in claim 6 in whichthe adherent, lubricous, protective coating additionally compriseshexagonal boron nitride in an amount of about 10 to about 30% by volumeuniformly distributed throughout the coating.
 8. A method as claimed inclaim 7 in which the protective coating additionally contains about 17%by volume hexagonal boron nitride uniformly distributed throughout thecoating.
 9. A method of protecting gas turbine components of titaniumalloy, which in operation at a temperature in the range of -60° to 600°C. have a first surface which is subjected to metal-to-metal wearagainst an opposed, contacting second surface of titanium alloycomprising: placing between said opposed, contacting first and secondsurfaces a lubricious, protective, thin film consisting essentially ofcobalt and at least one alloying element selected from the groupconsisting, by weight, of about 17 to about 35% chromium, about 3 toabout 12.5% aluminum, about 0.5 to about 5% silicon and about 0.3 toabout 2% yttrium for contact transfer of a lubricious, protectivecoating from the thin film to the opposed, contacting surface wherebythe surfaces are protected from metal-to-metal wear.
 10. A method asclaimed in claim 1 in which the protective coating is applied to one ofthe opposed contacting surfaces by thermal spraying, physical vapourdeposition or chemical vapour deposition.